(CNN) - An 11-year-old from Michigan said he was really going to wow his schoolmates with the "coolest" show-and-tell item anyone's ever brought to the sixth grade.

After all, it's not every day you get to show off a 13,000-year-old mastodon bone you and your cousin found in a stream behind your backyard.

"I thought it was a rock at first, but a couple minutes later I looked more at it, and I didn't think it was a dinosaur bone, but I wasn't sure," Eric Stamatin of Shelby Township Michigan told CNN on Thursday.

He and Andrew Gainariu, 11, from Troy, Michigan, were hunting for crayfish in the stream that extends from the middle branch of the Clinton River, as they often did, when they "got bored" and decided to build a dam.

They made an extraordinary discovery that June day.

John Zawiskie, a geologist and paleontologist at the Cranbrook Institute of Science confirmed in early November that what the "kids just being kids outside" discovered was not a rock at all, but an axis, a specialized second vertebrae behind the skull in the spinal column of an American mastodon.

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